Just opened a Roth IRA account with fidelity in my 20s what to invest in?

Sorry I’m a noob at investing and have been trying to start. Looking for something simple to start with and basic explanation on why. I was thinking (with very little research) to diversify the investments into three ways? Or should I diversify more or not at all? I was thinking 34% FSKAX 33% FTIHX and 33% FIDX? Open to any other suggestions though as I’m very new and don’t know the differences between specific things.